Electric heating element



Feb. 18, 1941. B. HUBALD 2 2,232,327

ELECTRIC Burma ELEMENT Filed Dec. 29, 19s? "///////4 gmmnnn ////////AZ Bernhard Hubadd lnvenlor By His Affornys Patented Feb. 18, 1941 UNITED STATES 2,282,327 ELECTRIC HEATING ELEMENT Bernhard llubald, Bitterfeld,

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Application December es, mi, sum No. 182,182

In Germany December 81, 1986 s Claims. (01. 13-25) 'ihis invention relates to improvements in means for supplying current to electric heatina ehments disposed in furnaces and consisting of carbon. graphite and the like.

5 In present day electric furnace practice, current is generally supplied to heating elements of graphite, carbon or the like. disposed inside the furnace, from thecurrent terminals located externallyofthefurnacebypassingthroughthe furnace wall to the actual heating elements, rods of either carbon or graphite, these rods frequently also forming part of the heating element itself. and, in any case being, of considerable diameter in order to avoid excessive heating of said rods.

15 Nevertheless it is still often 11 to watercool the mount for the connection to the external current supply line. Where rods of metal are employed for leading in the current, it has hitherto been necessary to cool the metal rod up 20 to within the hot zone so as to avoid a softening or even melting of the same. This cooling results in considerable amounts of useful heat being lost by conduction to the outside, such loss of useful heat being still further increased by the B5 furnace brickwork snugly surrounding the lead-in. In consequence of the intense heating of the ends of the leading-in rods inside the furnace, disturbances in the operation of the furnace resultlo ing from premature wear at the point of connection and, therefore. uncertain contact occur after thefurnace has been working a short time.

The present invention aims at obviating the f regoing drawbacks by using as material for the lead-in rods a metal or a metal alloy having a melting point lying above the temperature reached by the heating element.

In carrying out the invention, chrome-nickel heat resistant alloys have givengood results at temperatures up to about 1200' Or Iitheleadsaretobeheateduptohigher tempera ures, y p to about 2000 0., molybdenum may be used therefor according to the invention, and for still higher temperatures the leads may be made of tantalum or tungsten. Connection of the leads to the heating elements and the current terminals located outside the furnace is made in known manner, for example by screwing together the contact ends, which as far as the connection i with the heating element is concerned, affords the particular advantage that the end of the current leading-in rod. which is in close contact with the carbon forming the heating element, is hardened by the formation of resistant carbides and slightly increased in volume, thus ensuring a yet more perfect contact by cutting into the thread of the heating element. 7

It has been ascertained that. when using a graphite heating element to which the heating current is supplied by means of molybdenum rods 5 of a diameter of 1 cm., the heat losses, which are tantamount to an increased heating current consumption, amount to only about one tenth of the heat losses occurring when using g aphite rods 4 cm. in diameter under otherwise equal working 10 conditions. The advantages of the invention in respect of the conduction of heat to the outside are at once when it is borne in mind that the heat conductivity of molybdenum is hardly greater than that of graphite, while the electrical 1 conductivity of molybdenum is about twenty times as great as that of carbon. the latter circumstance enabling the use of current leads of much smaller diameter, and correspondingly smaller flow of heat for a given supply of heating current. At the same time the area of surface contacting with the furnace brickwork is also smaller and the losses of heat occasioned thereby are also reduced. As a further advantage of the invention it dispenses with the cooling arrangement for leadingin rods, of the metals hitherto employed, inside the furnace, which could be fitted only with dimculty.

In the annexeddrawing there is shown. in a purely ditic manner, and by way of example only, a suitable embodiment of the invenhi the drawing. it represents the furnace walls of ceramic material. b is a resistor element made of carbon (graphite), with both ends held by the leads c made of a metal or alloy of high melting point and high electrical conductivity. The ends of these leads contacting with the carbon resistor element. are connected therewith by means of screw di, and thread 42, respectively. e represents the current supply line outside the furnace, the current terminals being connected in the usual manner with the metallic leads c by means of nuts I.

I claim:

1. An electric furnace comprising a substantially closed heating chamber surrounded by walls. a resistor heating element substantially in the current leading-in means of minor diameter and consisting of a metal capable of forming a carbide with the carbon heating element and having a high melting point and high electrical conductivity, such means passing from the outside of said chamber through said wall into the inside of said chamber, the inner end of said means fitting snugly into a recess in the end of said carbon rod.

2. An electric furnace comprising a substantially closed heating chamber'surrounded by walls, a resistor heating element substantially in the form of a compact rod of carbon of major diameter which rod is disposed'in said chamber so as to radiate heat freely therein, and rod-shaped current leading-in means of minor diameter and consisting of molybdenum, such means passing from the outside of said chamber through said wall into the inside of said chamber, the inner end of said means fitting snugly into a recess in the end of said carbon rod.

8. An electric furnace comprising a substantially closed heating chamber surrounded by walls,

a resistor heating element substantially in the form of a compact .rod of carbon of major diameter which rod is disposed in said chamber so as to radiate heat freely therein, and rod-shaped current leading-in means oiv minor diameter and consisting of tungsten, such means passing from the outside of said chamber through said wall into the inside of said chamber, the inner end of said means fitting snugly'into a recess in the end of said carbon rod.

4. An electric furnace comprising a substantially closed heating chamber surrounded by walls, a resistor heating element substantially in the form of a compact rod of carbon of major diameter which rod is provided with bores at both its ends, said bores being further provided with internal screw threads, and which rod is disposed in said chamber so. as to radiate heat freely therein, and rod-shaped current leading-in means of minor diameter and consisting of a metal capable of forming a carbide with the carbon heating element and having a high melting point and high electrical conductivity. such means passing from the outside of said chamber through said wall into theinsideofsaidchambentheinnerendofsaid means being provided with screw threads so as to register with the screw threads provided in the ends of said carbon rod.

5. An electric furnace comprising a substantially closed heating chamber surrounded by walls, a resistor heating element substantially in the form of a compact rod of carbon of maior diameter which rod is provided with bores at both its ends, said bores being further provided with internal screw threads, and which rod is disposed in said chamber so as to radiate heat freely therein, and rod-shaped current leading-in means of minor diameter and consisting of molybdenum, such means passing from the outside of said chamber through said wall into the inside of said chamher, the innerend of said means being provided with screw threads so as to register with the screw threads providedin the ends of said carbon rod.

6. An electric furnace comprising a substantially closed heating-chamber surrounded by walls, a resistor heating element substantially in the form of a compact rod of carbon of major diameter which rod is provided with bores at both its ends, said bores being further provided with internal screw threads, and which rod is disposed in said chamber so as to radiate heat freely therein, and rod-shaped current leading-in means of minor diameterand consisting of tungsten, such means passing from the outside of said chamber through said wall into the inside of said chamber, the inner end of said means being provided with screw threads so as to register with the screw threads provided in the ends of said carbon rod.

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